Corn (Zea mays L.) is a monoecious plant, i.e., the male and female flowers develop on the same plant. They are located on the tassel and ear, respectively. Each silk on the ear represents an individual female flower, and each kernel represents a separate pollination event. Natural pollination occurs when pollen falls from the tassel onto the silk of the same plant, or is carried by wind from the tassel of one plant to the silk of a neighboring plant.
Corn breeders have employed controlled pollination, artificial selection, and genetic analysis to develop numerous genetic lines or varieties of corn that display desired traits such as yield potential, maturity time, disease resistance, insect resistance, ear size, plant height, drought tolerance. Established lines have been used as starting material for further rounds of crossing, selection, and analysis, to develop new and different varieties that display enhancement of particular traits or new combinations of traits. See, for example, Principles of Cultivar Development, Vol. 1, Fehr, W. ed. pp. 315-376, Macmillan, New York (1982).
The totality of the observable traits of a corn plant, i.e., the phenotype, results from the presence and interaction of many thousands of individual genetic loci. Each locus includes a pair of alleles, i.e., one from each parent. When a plant contains different alleles at a large number of loci, the plant is said to be heterozygous. In accordance with classical (Mendelian) genetics, a cross between two heterozygous plants yields a highly heterogeneous (nonuniform) population of offspring. Thus, heterozygous plants are not "true-breeding." However, crosses of heterozygous plants are useful starting material for creation of new inbred, i.e., highly homozygous, lines, which are true-breeding.
Creation of a new inbred line can begin with selection of individual lines or populations judged superior with respect to one or more traits of interest. The genetic backgrounds of the selected plants are combined by crossing to create a gene pool upon which selection for desired traits may be practiced. Selected progeny plants are self-pollinated and plants in the next generation that exhibit the desired phenotype(s) are selected from further selfing. This process can be repeated for several generations (typically 5-8 generations designated F.sub.1, F.sub.2, F.sub.3, etc.), until the desired degree of homozygosity is achieved.
Pedigree breeding and recurrent selection breeding are two methods that can be used to develop inbred lines from breeding populations. Pedigree breeding typically begins with the crossing of two different genotypes, and superior plants are selfed and selected in successive generations. In pedigree breeding, five or more generations of selfing and selection typically is practiced. Recurrent selection breeding can be used to improve an inbred line, e.g., to transfer a specific desirable trait from one germplasm source to an inbred that lacks the trait.
Inbred lines, however made, typically display relatively poor growth and vigor. Nevertheless, two (or more) different inbred lines can be crossed to produce a heterozygous hybrid that displays growth and vigor superior to that of either inbred parent line. This phenomenon is known as hybrid vigor or heterosis. Because of hybrid vigor, practically all corn produced in the United States is grown from hybrid seeds. In some cases, hybrid seeds are produced from controlled crosses of three or even four different inbred lines.